1. Field of the Invention
A certain aspect of the present invention relates to an imaging device.
2. Description of the Related Art
Imaging devices are popularly used as information obtaining units, for example, in portable equipment, on-vehicle equipment, medical equipment, and industrial equipment. There is a growing demand for a thinner and smaller imaging device that can be installed in any apparatus or any place. Also, it is an essential feature of an imaging device to be able to take an image that is in focus (hereafter called an in-focus image) and thereby to obtain accurate information. To obtain an in-focus image at any given subject distance, it is necessary to focus the lens at the subject distance. The focusing function is particularly important when taking a close-up image at a short subject distance because the depth of field becomes small and the image quality varies greatly according to the change of the subject distance.
As an example of a related-art technology for focusing the lens at every subject distance, Japanese Patent Application Publication No. 2005-165058 discloses an automatic focusing technology where the arrangement of lenses is changed according to the subject distance. However, to implement the automatic focusing function, it is necessary to provide multiple lenses and a mechanism for changing the positions of the lenses relative to each other. This in turn complicates the configuration of an imaging device and increases the size and costs of the imaging device.
Meanwhile, Japanese Patent Application Publication No. 2006-251613 and Japanese Patent Application Publication No. 2008-521043 disclose a liquid lens (electrowetting lens) and a liquid-crystal lens the radii of curvature of which can be changed by applying a voltage. However, these lenses consume a large amount of power. Also, since these lenses are made of fluid and movable, they have low durability and low environmental resistance.
As still another example of a related-art technology, Japanese Patent Application Publication No. 2007-158825 discloses a method for obtaining an image that is in focus at multiple subject distances. In this method, a lens array composed of multiple lens sets with different focal lengths is used to obtain a compound-eye image, pixels at specific positions are extracted from the compound-eye image, and a single-eye image is reconstructed from the extracted pixels. In this method, however, since the back focal lengths of the lenses constituting the lens array are not defined, the lenses have different imaging distances and optical magnifications and form single-eye images with different sizes. This in turn complicates a process of extracting an in-focus single-eye image or pixels from an obtained compound-eye image and a process to be performed on the extracted in-focus single-eye image or pixels.